The sol-gel method is known as one of the methods for producing inorganic oxide particles such as silica, titania, zirconia or alumina particles. This production method is to obtain inorganic oxide particles by the hydrolysis reaction and polycondensation reaction of a metal alkoxide such as tetraethoxysilane in an organic solvent containing water in the presence of an acidic catalyst or a basic catalyst. The sol-gel method is characterized in that fine inorganic oxide particles which are spherical and relatively uniform in particle size are obtained.
In a dispersion of inorganic oxide particles obtained by the reaction of the sol-gel method, the inorganic oxide particles are highly dispersed as fine primary particles. Therefore, it is extremely difficult to collect a cake (a concentrate of a dispersion containing inorganic oxide particles in high concentration) of inorganic oxide particles from this dispersion by filtration.
Then, to collect the cake from the above dispersion of inorganic oxide particles, for example, a method in which a dispersion medium is removed by heating and/or depressurizing the dispersion or a method in which the dispersion is centrifuged to spin down the inorganic oxide particles and then decantation is carried out is employed. The inorganic oxide particles can be obtained by drying the cake obtained by the above method to remove the dispersion medium remaining in the cake.
However, in the case of the above cake collection step, a firmly agglomerated product of the inorganic oxide particles contained in the cake is formed, and further a firm agglomerate of the inorganic oxide particles is formed by the subsequent drying step. Therefore, even when the inorganic oxide particles obtained by the above method are disintegrated, fine inorganic oxide particles which are uniform in particle size are not obtained. When the inorganic oxide particles are re-dispersed into a dispersion medium such as a resin or a solvent, the above agglomerate is not easily disintegrated by the shear of a disperser and not uniformly dispersed in a dispersion medium. Especially in the case of fine inorganic oxide particles having a primary particle diameter of 1 μm or less produced by the reaction of the sol-gel method, the cohesion force of the particles is very strong and an extremely firm agglomerate is apt to be formed.
To solve the above problem, various proposals have been made up till now.
For example, JP-A 6-115925 proposes a method in which inorganic oxide particles obtained by drying a dispersion of inorganic oxide particles are disintegrated by a jet mill. According to this method, it is possible to obtain inorganic oxide particles which are uniform in particle size to some extent but the number of firm agglomerates formed at the time of drying cannot be reduced. Therefore, it cannot be said that this is not the technology for basically solving the above problem.
JP-A 2003-277025 discloses a method for preventing agglomeration by adding ethylene glycol to a dispersion of inorganic oxide particles while a dispersion medium is removed from the dispersion so as to solve problems such as the agglomeration and sintering of a metal oxide without disintegration. According to this method, the formation of an agglomerate which is hardly disintegrated is suppressed but ethylene glycol may remain in the inorganic oxide particles unless the drying and calcination conditions are controlled precisely. Therefore, it is difficult to adopt this method according to the application purpose of the inorganic oxide particles.
Printing is becoming faster and picture quality is becoming higher in electrophotographic technologies for copiers and printers. Since the number of times of toner transfer increases in the copiers and printers, an additive for enhancing the transfer efficiency of a toner is becoming necessary. It is reported that sol-gel silica (surface treated inorganic oxide particles) which has a narrow particle size distribution and a hydrophobilized surface is effective as the above additive (JP-A 2002-108001).
To produce the sol-gel silica having a hydrophobized surface, there is proposed a method for obtaining hydrophobized silica by adding a silazane compound to silica sol to react it with the silica sol (JP-A 2006-151764). The collection of a cake of silica after hydrophobization in this method is carried out by distilling off a solvent from a dispersion containing surface treated silica. Although this collection method can be easily employed when silica is taken out from a flask in a laboratory scale, when this method is applied to an actual large-scale production plant, it is difficult to scrape out a cake of the hydrophobic sol-gel silica from a reaction oven, which is not practical. Although it is conceivable that a filtration method is employed to collect silica after hydrophobization, when the filtration method is employed to collect fine particles having a particle diameter of 1 μm or less such as the particles produced by the sol-gel method, the particles pass through a filter paper or a filter cloth. When the solvent is removed from the particles after filtration to dry the particles up, the hydrophobic sol-gel silica firmly agglomerate, whereby it may be difficult to re-disperse it.